1. Field of the Invention
The present invention relates to an electrostatic accelerator (Hereinafter, referred to as “electrostatic acceleration column”) for accelerating or decelerating charged particles, and more relates to an ion implanting apparatus provided with the electrostatic acceleration column.
2. Description of the Related Art
One example of an ion implanting apparatus provided with a conventional electrostatic accelerator (Hereinafter, referred to as “electrostatic acceleration column”) for accelerating or decelerating ions (positive ions in the following, except where specifically noted), which are a kind of charged particles.
This ion implanting apparatus is constructed as follows: ions 4 (to be more specific, an ion beam) are emitted from an ion source 2. A desired kind of ions is selected by a mass separating magnet 6. Then, the ions are accelerated or decelerated by an electrostatic acceleration column 10 to have desired energy. The desired kind of ions 4 having desired energy is implanted into a target (for example, a substrate of a semiconductor wafer or the like) held by a holder 16 to perform ion implantation. The conventional electrostatic acceleration column 10 has a plurality of electrodes 11 arranged in a straight line in the traveling direction of the ions 4.
In this respect, a path of the ions 4 from an exit of the ion source 2 to the holder 16 is received in a vacuum container but the illustration thereof is omitted here. Moreover, there are cases where a scanner for scanning the ions is provided but its illustration is also omitted here. This is the same in FIG. 4.
In the ion implanting apparatus described above, while the ions 4 are traveling, for example, from the exit of the mass separating magnet 6 to the exit of the electrostatic acceleration column 10, the ions 4 emitted from the electrostatic acceleration column 10 are mixed with ions or neutral particles having energy different from desired energy by the phenomenon described below. This kind of mixing phenomenon is called “energy contamination”.
(1) Part of the ions 4 are converted into neutral particles by a charge conversion caused when the ions 4 collide with residual gas including ions or neutral particles having energy different from desired energy. The neutral particles are not accelerated or decelerated by the electrostatic acceleration column 10, which is different from the ions 4. Thus, the ions 4 are mixed with the neutral particles having energy different from desired energy.
(2) Part of the ions 4 are changed in valence by the charge conversion caused when the ions 4 collide with the residual gas. When the valences of the ions 4 are changed, the ions 4 accelerated or decelerated by the electrostatic acceleration column 10 become different from each other in energy because voltage applied to the electrostatic acceleration column 10 is constant.
For example, when a divalent ion is changed into a univalent ion, the energy of the ion accelerated by the electrostatic acceleration column 10 becomes one half. In this manner, the ions 4 are mixed with ions having energy different from desired energy.
(3) In a case where the ion 4 is a molecular ion, the ion 4 is changed into an ion different from the original ion by molecular dissociation. For example, a BF2 ion is dissociated into a BF ion and a F ion, or a B ion and a F2 ion. The energy of each ion produced by the molecular dissociation becomes smaller than the energy of the original ion. This is because the sum of these energies is equal to the energy of the original ion. In this manner, the ions 4 are mixed with ions having energy different from the desired energy.
It is not desirable in a case of utilizing charged particles such as ions that the energy contamination described above occurs. For example, in the ion implanting apparatus, it is not desirable that ions having desired energy and ions having energy different from the desired energy or neutral particles are implanted into the target 14. This is because the ions having different energies change implantation depth and implantation distribution from those at an initial stage and thus degrade ion.
Then, in order to suppress the energy contamination, in a conventional technique, like the example shown in FIG. 5, an energy separating unit 12 for selecting and passing ions having specific energy (that is, having an energy separating function) is disposed on the downstream side of the electrostatic acceleration column 10 and only ions having the desired energy are selected by this energy separating unit 12. To be specific, this energy separating unit 12 is a deflecting magnet or an electrostatic deflecting unit.
When the energy separating unit 12 described above is disposed, however, it increases the transport distance of the ions 4 by the amount relating thereto and thus decreases the transport efficiency of the ions 4. This is because when the transport distance increases, divergence of the ions 4 in the transport caused by space charges increases, which results in increasing loss of the ions 4. The same holds true for a case where charged particles other than ions are transported.